quico-os-setup/zfs-pacman-hook
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docs(script): Our hooks go first before kernel and modules are touched (#1)

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hygienic-books 2023-05-06 19:51:29 +02:00
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@ -15,12 +15,12 @@ Get started like so:
1. Symlink to files, for example 1. Symlink to files, for example
``` ```
sudo ln -s <repo>/pacman-zfs-snapshot.sh /usr/local/bin/pacman-zfs-snapshot sudo ln -s <repo>/pacman-zfs-snapshot.sh /usr/local/bin/pacman-zfs-snapshot
sudo ln -s <repo>/pacman-zfs-snapshot-install.hook /usr/share/libalpm/hooks/pacman-zfs-snapshot-install.hook sudo ln -s <repo>/pacman-zfs-snapshot-install.hook /usr/share/libalpm/hooks/00-pacman-zfs-snapshot-install.hook
sudo ln -s <repo>/pacman-zfs-snapshot-remove.hook /usr/share/libalpm/hooks/pacman-zfs-snapshot-remove.hook sudo ln -s <repo>/pacman-zfs-snapshot-remove.hook /usr/share/libalpm/hooks/00-pacman-zfs-snapshot-remove.hook
sudo ln -s <repo>/pacman-zfs-snapshot-upgrade.hook /usr/share/libalpm/hooks/pacman-zfs-snapshot-upgrade.hook sudo ln -s <repo>/pacman-zfs-snapshot-upgrade.hook /usr/share/libalpm/hooks/00-pacman-zfs-snapshot-upgrade.hook
sudo ln -s <repo>/pacman-zfs-snapshot.conf /etc/pacman-zfs-snapshot.conf sudo ln -s <repo>/pacman-zfs-snapshot.conf /etc/pacman-zfs-snapshot.conf
``` ```
Note that while you may choose arbitrary locations for symlinks the `pacman-zfs-snapshot-*.hook` files reference `/usr/local/bin/pacman-zfs-snapshot`. Change that accordingly if you need to. Note that while you may choose arbitrary locations for symlinks the `00-pacman-zfs-snapshot-*.hook` files reference `/usr/local/bin/pacman-zfs-snapshot`. Change that accordingly if you need to.
1. For datasets you want auto-snapshotted add property `space.quico:auto-snapshot=true` 1. For datasets you want auto-snapshotted add property `space.quico:auto-snapshot=true`
``` ```
zfs set space.quico:auto-snapshot=true zpool/root/archlinux zfs set space.quico:auto-snapshot=true zpool/root/archlinux
@ -30,9 +30,13 @@ Get started like so:
# What's it do? # What's it do?
In `pacman` on every `PreTransaction`, meaning right before any actual operation on a package begins, we trigger a ZFS snapshot. By default we identify the active system dataset by doing `findmnt / --noheadings --output source`. If exactly one source returns that is the exact name of a ZFS dataset in an imported zpool we create a snapshot on it. If no source returns we silently exit. If more than one source returns we raise an error and halt the `pacman` transaction. In `pacman` on every `PreTransaction`, meaning right before any actual operation on a package begins, we trigger a ZFS snapshot. This happens via a so-called hook which is a plain text config file. Hook files make use of the Arch Linux Package Management (ALPM) library, also known as `libalpm` for which `pacman` is a frontend. By default hooks are stored in `/usr/share/libalpm/hooks`. Additionally `/etc/pacman.conf` has a directory configured as:
```
#HookDir = /etc/pacman.d/hooks/
```
Hook files from both directories are collectively parsed and executed in lexicographical order. Hook names from _this_ repo begin with `00-*` so on a default Arch Linux they are the first to be executed during `pacman` transactions.
We retain two different snapshot chains, one for `pacman` transactions that only affect what we are calling _trivial_ packages and a separate chain for _important_ packages. By default only the exact regular expression package name match `^(linux|systemd|zfs-(dkms|utils))$` is considered important. Whenever an important package is affected by a transaction a snapshot goes into the corresponding chain. In all other cases - when an important package is not affected - snapshots go into the trivial chain. For ZFS snapshots intended to save your bacon the `00-*` naming convention is particularly critical. In `/usr/share/libalpm/hooks` you can see for example that when a kernel upgrade happens `60-mkinitcpio-remove.hook` is executed (deleting your existing `vmlinuz-*` kernel image for example at `/boot/vmlinuz-linux`). After that if you're using the `zfs-dkms` package which itself requires `dkms` which in turn installs `71-dkms-remove.hook` this hook removes your ZFS kernel module files. Both the `60-*` and optionally the `71-*` hook (for `zfs-dkms` users) run early due to their naming. If we don't create a snapshot before them we end up creating a snapshot that has not kernel image and no ZFS kernel module files. Out `00-*` hook files are executed first ensuring that a snapshot can safely return you to a working system.
The _trivial_ snapshot chain by default keeps 15 snapshots, the _important_ chain keeps 5. The thought process here is that you will likely not futz around with a kernel every day whereas you may very well install arbitrary packages multiple times a day. Snapshots should keep you safe for a couple of days hence the defaults of 5 and 15 snapshots, respectively. The _trivial_ snapshot chain by default keeps 15 snapshots, the _important_ chain keeps 5. The thought process here is that you will likely not futz around with a kernel every day whereas you may very well install arbitrary packages multiple times a day. Snapshots should keep you safe for a couple of days hence the defaults of 5 and 15 snapshots, respectively.